ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
387 
First we have the wide dissimilarity of their empty shells from those 
of any living types ; next their rigid isolation to a solitary great lake of 
enormous age ; next their wonderful similarity to those left fossilised 
at the bottom of the old Jurassic seas ; and lastly their morphological 
characters like those of incompletely developed embryos. 
Dimorphism in Molluscs.* — Dr. E. G.J Conklin publishes some 
interesting observations on variations in form in the species of Crepi- 
dula. In the large and sedentary species, such as C. ft mica! a and 
C. plana, the shape of the shell varies according to the surface upon 
which the animal fixes itself, but there is no evidence that these modi- 
fications of form are inherited. In the last-named species, C. plana , 
there is a constant dimorphism which cannot be directly referred to a 
mechanical factor. The species occurs in shells inhabited by Eapagurus 
Eernhardus, and also in those inhabited by the little hermit crab Eupa- 
gurus longicarpus. In the latter case the individuals are dwarfs, having 
a body volume which is only 1/13 of that of the other more typical form. 
Examinations showed that the size of the cells is the same in the two 
forms, but the dwarfs contain fewer cells than the other, and produce many 
fewer eggs. The difference is not inherited — the two forms are physio- 
logical and not morphological varieties, but the author believes that the 
differences are not dependent upon pressure or upon relative supplies 
of food and oxygen, but on some other unknown factor. Further, the 
same species of Crepidula shows marked sexual dimorphism, the female 
being much (about 15 times) larger than the male. The males retain 
throughout life the power of locomotion possessed by the females only 
in the young stage. 
The author’s special point is that the factor which determines size 
does so by inhibiting cell-growth and division, and that for Crepidula 
at least the explanations of the cessation of growth put forward by Semper 
and De Yarigny respectively are not valid. 
Primitive Kidney of Pulmonates'.'j' — Dr. Johannes Meisenheimer 
has investigated this structure in the embryos of members of both 
the Basommatophora and the Stylommatophora, and finds that, different 
as the organ appears in the two groups, it can in both cases be reduced 
to a common type. This type consists of a simple tube ending in- 
ternally in a ciliated cell ; from this typical form divergence takes jdace 
in two directions. In the Basommatophora the number of cells is limited 
to four, and of these one, the giant-cell, is the true excretory cell, the 
other cells being probably of importance only in the transportation of 
the excretory products. The ciliated cell also retains its primitive ex- 
cretory function, but is chiefly of importance in facilitating the outward 
current by means of its cilia. In the Stylommatophora specialisation is 
chiefly shown in the great increase in number of the cells which consti- 
tute the tube, and which are all excretory in function. The ciliated 
cells also increase in number, and gradually lose most of their excretory 
importance. No great importance is to be attached to the fact that in 
the Basommatophora the canal of the kidney is intracellular, while in 
Stylommatophora it has a distinct epithelial lining. The primitive 
* Proc. Acad. Nat. Sci. Philadelphia, 1898, pp. 435-14 (3 pis.). 
t Zeitschr. f. wiss. Zool., lxvi. (1899) pp. 709-24 (1 pi. and 4 figs.). 
